1. Field of the Invention
This invention relates generally to an amplitude modulation (AM) receiver, and more particularly to a novel AM receiver provided with an automatic gain control (AGC) amplifier.
2. Description of the Prior Art
A silicon diode or germanium diode is normally used as an AM detector in a transistorized AM receiver. As is well known, germanium diodes have a relatively low threshold voltage of about 0.1V. Accordingly, even when low level signals are received, the rectifying efficiency of the germanium diode is good and there is little distortion. However, low level signals received include a large noise component, which cannot be neglected. As a result, the output signal of the germanium diode includes a large noise component.
Silicon diodes have a relatively high threshold voltage of about 0.6V. Accordingly, when low level signals are received, the rectifying efficiency of silicon diodes is deteriorated. However, the noise component of the detected output signal of silcon diodes is small. The fact that the rectifying efficiency is less when low level signals are received corresponds to the fact that the AM detecting output is distorted.
Generally, the intermediate frequency amplifying stage of the AM receiver includes an automatic gain controlled (AGC) amplifier which is controlled so as to reduce its gain when stronger signals are received. As a result, distortionless reproduced sounds can be reproduced by a loudspeaker. On the other hand, the gain of an AGC amplifier is not controlled when weaker signals are being received. As a result, reproduced sounds with a high noise component, and therefore with a poor signal-to-noise ratio (S/N ratio), or with great distortion factor are reproduced by the loudspeaker.